Doesn't the id refer back to Aemilius avunculus, so shouldn't it be the masculine 'is' instead of the neuter 'id'?

Good observation, but the intended meaning here is not to give more information about Aemilius, but to explain the meaning of the word "avunculus". In English we would translate it as "Aemilius is [an/the/his/her] uncle, that is the mother's brother" (not "he is the mother's brother").

In fact, Jowens, what you are looking at here, 'id est', is the full length version of the commonly used i.e., 'that is to say', which does not provide an example, as many people think, but a clarification, or expansion of a point.

The only thing we can guarantee when communicating via the internet is that we will be almost completely misunderstood, and likely cause great offence in doing so. Throw in an attempt at humour and you insure a lifelong enemy will be made.

In addition to the other great replies, I think it is worth mentioning, from a cultural standpoint, that Iulius is defining the word avunculus for his children. I think this is a very likely sentence which must have been used in one form of another millions of times by Roman parents throughout the generations to teach their children the remarkably complex kinship terminology of Latin.

How does this relate to the original question?

In reference to the original question, It is grammatically correct to write “Aemilius avunculus vester est, is est frater matris,” and it would translate very well into English, too: “Aemilius is your uncle, he is your mother’s brother;” however, as I mentioned above, Roman kinship terminology is remarkably sophisticated. The sentence with is isn't likely to occur in authentic Latin because avunculus can only mean the mother’s brother. If Aemilius had been the father’s brother, he would have said “patruus” rather than “avunculus.” In short, the alternate version with is would be redundant.

D. Q. Dauthier wrote:The sentence with is isn't likely to occur in authentic Latin because avunculus can only mean the mother’s brother. If Aemilius had been the father’s brother, he would have said “patruus” rather than “avunculus.” In short, the alternate version with is would be redundant.

I think there's a non sequitur there, D.Q. (Welcome, BTW.) I think it's grammatical and proper to pose the question "Who is your uncle on your mother's side?" and to follow with "Aemilius is your uncle on your mother's side. He is your mother's brother."

Not sure, I still think it's redundant, although proper and grammatical. You are thinking in English, where the word "uncle" is vague. However, the explanation of relationship is possible when explaining to children. Both 'is' or 'id' could be used, if that's your point.

joels341 wrote:However, the explanation of relationship is possible when explaining to children. Both 'is' or 'id' could be used, if that's your point.

Of course it is, when explaining to anyone, not just to children. What's ambiguous about the following?Id certum est, cuicunque in explicando, non solùm liberis. Quid ambigui est hoc?.

adrianus wrote:I think it's grammatical and proper to pose the question "Who is your uncle on your mother's side?" and to follow with "Aemilius is your uncle on your mother's side. He is your mother's brother."